He probably has the legal authority, but it’s still a bad precedent.

With Democrats in Congress refusing to appropriate $5.7 billion for a border wall, President Trump now says he may declare a national emergency. He’s probably right that he has the legal authority, but it would set a bad precedent that conservatives who believe in the separation of powers could live to regret.

Mr. Trump may declare an emergency that would let him reallocate funds that Congress has appropriated for military construction to build his wall. Before visiting the border on Thursday, he warned that if negotiations...